[Applied field in industry]
This invention relates to the structure of a ladder-type ceramic filter, and more particularly to a ladder-type ceramic filter in which resonators having different characteristics are arranged.
[Prior art and the problem therewith]
There are used various ladder-type ceramic filters having a high selectivity filter characteristic in which a plurality of resonators consisting of an element made of a piezoelectric material such as barium titanate, are connected in the form of a ladder.
Such ladder-type ceramic filters are constituted by arranging a plurality of resonators connected in series and a plurality of ones connected in parallel between the input and output terminals.
In general, for series and parallel resonators, there are employed resonators having resonant frequencies equal to each other and antiresonant frequencies equal to each other, respectively. In this instance, setting is made such that respective differences .DELTA.F between the resonant and antiresonant frequencies of the resonators are fixed.
For example, for three series resonators, setting is made such that the resonant and antiresonant frequencies are equal to 455 and 471 KHz, respectively, thus allowing the difference therebetween to be equal to 16 KHz. In addition, for three parallel resonators, setting is made such that the resonant and antiresonant frequencies are equal to 439 and 455 KHz, respectively, thus allowing the difference therebetween to be equal to 16 KHz which is the same value as the difference for the above-mentioned three series resonators. These resonators thus set are used.
The characteristic of the ladder-type ceramic filter thus constituted is shown in FIG. 3. As indicated by the curve 31, the ripple R is large and the pole width W of the attenuation is narrowed. When the ripple R becomes large, the distortion factor is deteriolated, failing to ensure the distortion of the adjacent channel unless the pole width W of the attenuation is sufficient.
As just described above, using resonators having uniform characteristics provides the merit that particular consideration is not taken for the arrangement of resonators, but is questionable to much extent in view of characteristics.